The present invention generally relates to an electrical resonator and more particularly, to a dielectric resonator to be employed, for example, as a filter.
Conventionally, in the dielectric resonator of this kind, an arrangement has already been known, for example, as shown in FIG. 1, in which the resonator R is composed of a dielectric resonator unit 3 which is columnar in shape, and is shielded with a conductive case 1 therearound. In the dielectric resonator R of the above described type, when a basic mode of the resonator unit 3 is TE.sub.01.delta. mode, it has been also known that a spurious response of TM.sub.01 mode or mode undesirably appears in the vicinity of a resonant point in TE.sub.01.delta. mode. Therefore, for example, when the resonator of this kind is employed for a filter circuit, it is required in obtaining a filter characteristics with high quality to shift the spurious response of HE.sub.11.delta. mode to a frequency zone considerably higher than the resonant point in TE.sub.01.delta. mode.
It has been, however, technically difficult to remove the spurious response in the vicinity of the above described resonant point in the resonator R of the conventional type having a columnar resonator unit 3 therein.
Therefore, it has been proposed that a resonator having a resonator unit which is cylindrical in shape is employed for the filter circuit instead of the resonator having a resonator unit of the columnar type. However, this has been still insufficient for obtaining a resonator with reliable filtering characteristics which is intended for practical use, although the spurious response of HE.sub.11.delta. mode can be shifted to some extent to a frequency zone slightly higher than the resonant point.